Microbial Pretreatment Of Biomass For Improving Biomethane Production Via Anaerobic Digestion

Lignocellulosic materials including crop residues such as rice straw(RS)and wheat straw(WS)are massively generated agricultural wastes in China,which serve as a potential feedstock for renewable energy production via anaerobic digestion(AD).However,their direct digestion without pretreatment leads to a low biodegradability and methane yield due to their complex structure.There are many advanced pretreatment technologies including biological,chemical and physical pretreatment,which have been tested at the laboratory scale for the efficient utilization of lignocellulosic biomass.Among these pretreatment methods,microbial pretreatment is regarded as a potential pretreatment technique for lignocellulosic material utilization in the most promising,cost-effective and environmentally friendly way.It is imperative to investigate the physiochemical changes occurring during the pretreatment,as they can help in developing effective models,which can be useful in future designing of the pretreatment processes.This study employs microbial pretreatment combined with oxygen supply to enhance the energy conversion potential of RS and WS in a more cost-effective and time efficient way.The oxygen was supplied at different dosages to establish a connection between suitable oxygen levels,microbial functions,and rate of hydrolysis.The results are stated below:First,five broad varieties of reagents rich in microbes such as liquid consortium(LC),cow manure(CM),sheep dung(SD),biogas slurry(BS),and straw-decomposing consortia(SC)have been used as a source of useful microbes combined with oxygen supply to explore the effect of varying oxygen concentrations on the functioning of these microbes.The results showed that RS pretreated by SC has given the highest cumulative methane yield(CMY)of 311.7 mL/gVS at 12 mLO2/gVS,followed by BS,CM,LC,and SD with the CMY of 270.5,263.2,257.1,and 256.7 mL/gVS at 6 mLO2/gVS,respectively.The RS pretreated by SC,BS,CM,LC,and SD led to the improvement in the methane yield by 88.7%,63.7%,59.3%,55.6%,and 55.4%,respectively,compared to the untreated RS.The economic,energy,and environment analysis confirmed that microbial pretreatment combined with oxygen supply is a promising technique to utilize RS for methane productionSecond,WS was used as another feedstock for AD and was pretreated using the same microbial reagents,and oxygen supply dosages while maintaining the same operational parameters.Results revealed that WS pretreated by SC,SD and BS have given the highest cumulative methane yield(CMY)of 316.2,264.6 and 255.8 mL/gVS at 12 mLO2/gVS,respectively,whereas WS pretreated by LC and CM gave the highest CMY of 279.2 and 198.1 mL/gVS at the 6 mLO2/gVS,respectively.The WS pretreated by SC,LC,SD,BS,and CM have exhibited the improvement in the methane yield by 78.4%,57.6%,49.3%,44.4%,and 11.8%,respectively along with enhanced biodegradability as compared to the untreated WS.The structural analysis using SEM,FTIR and XRD showed that the pores and structures of WS were greatly ruptured,along with decrease in the crystallinity index during pretreatment.Third,microbial community analysis for RS and WS was carried out to study the microbial community changes after the microbial pretreatment and after AD,with and without oxygen supply.After microbial pretreatment of RS and WS,Firmicutes,Bacteroidetes,and Proteobacteria were the predominant phylums without oxygen supply,however,oxygen supply further enhanced the RA of these phylums.After AD,the microbial community analysis of RS and WS at genera level revealed that the relative abundance(RA)of Clostridium Ⅲ,which facilitates cell wall degradation by following hydrolytic pathways along with other sugar fermenting bacteria,such as Saccharofermentans,increased for RS and WS pretreated by microbial reagents with and without oxygen supply as compared to untreated.Interestingly,these predominant bacterial genera belong to phylum Firmicutes,which might have led to improved rate of hydrolysis and consequently enhanced the methane production.Archaeal community analysis in both pretreated RS and WS with and without oxygen supply showed a shift in acetoclastic and hydrogenotrophic methanogens,indicating the precursor changes,resulting from pretreatments and varied microbial metabolism,which might have enhanced the methane production performance of both substrates.Hence,for RS and WS,the microbial pretreatment combined with oxygen supply has positively influenced the methane yield.Fourth,biochar was used as an additive in order to explore the possible role of biochar on the improvement of methane production performance of RS and WS.Two types of biochars namely discarded mushroom stick biochar(DMB)and Robinia pseudoacacia biochar(RPB)pyrolyzed at different temperatures of 500℃ and 800℃ were introduced into the AD system.Results showed that the improved yield with both biochars for RS and WS was in the range of 24.0 to 29.7%and 19.7 to 24.6%,respectively,as compared to the control groups of RS and WS.The highest CMY of 302.3 mL/gVS was obtained for RS by adding RPB800℃,followed by WS 290.8 mL/gVS by adding DMB800℃.The SEM images revealed microbial adhesion on the surface of the biochar due to its high surface area,which might have played a key role in accelerating the direct interspecies electron transfer process and resulting in overall enchantment in the methane production of RS and WS.The findings of this study can be helpful in designing efficient biological strategies by emulating microbial ecology in AD digesters to give more promising solutions to enhance the degradation of complex lignocellulosic materials for methane production.This study can be used as a reference for future utilization of the diverse range of lignocellulosic materials for clean energy production at industrial scale.